blades to generate maximum thrust. Using a reduction unit allows a large propeller to operate in the proper RPM range by slowing the blades down, preventing the fan from pushing too fast at idle speed, or exceed its maximum RPM rating.
The actual length of the fan blades, the width of the blades, the number of blades on the fan, and the pitch (angle) of the fan blades when installed in the hub are all elements that deserve attention and will affect fan performance. It’s best to contact a company that works with bow-fishermen regularly, as they will have the knowledge and experience to get your fan setup for the best performance with the engine you choose. Companies that have this knowledge include Powerfin, Arrowprop, Ivoprop and Competition Aircraft.
The best setups maximize the amount of thrust produced for the size of engine and fan combi-nation.
When you have sourced out your fan parts it’s important to
build the protective shroud around the fan to be sturdy enough to absorb an impact. If the shroud were to flex, or bend into the path of the blades the results would be catastrophic. Also, flexing metal will fatigue over time, or rattle annoyingly.
Steel, stainless steel, or alum-inum are all common choices. Steel has the advantage that it’s easy to work with and less expensive, but it can be heavy. Stainless steel is nice if you’re looking to avoid corrosion, but you’ll need to take extra steps to weld it and it can be pricey. Aluminum is lighter in weight, but it will need to be braced sufficiently to avoid flexing and may require more material than a steel shroud.
There are great stresses placed on the steering shafts and pede-stals, so great attention should be paid to these areas. The steering shaft is typically a solid steel rod to absorb the stress and avoid bending/cracking, plus it is easy to weld to when employing cer-tain steering systems.
Mounting the motor to the
shroud and connecting the fan should all be mocked up prior to the final installation. If using a reduction unit it will be necessary to have the motor mounted to the shroud lower than the fan by the proper spacing depending on the size of the belt or gear box employed. This configuration keeps the weight of the engine and the center of gravity lower.
(continued on next page)
Photo by Anthony Crooks
Building the fan stand to be rugged and to accept the weight and forces the fan produces will prevent breakage and minimize lost thrust due to flexing.
Pusher Fans (continued from page 8)
9
Photos courtesy of Tim Escott - www.tjedesigns.com
Steering systems are usually made up of drum and cable, chain and sprocket, or actuator driven components. Here is a chain and sprocket set up. Keeping the sprocket ratio at 1:1 between the front and the back will allow you to know which direction the fan is pointing by the way the steering stick is pointing. Bearings are used to hold the shaft in place and to allow it to turn freely. Drums work similarly, but are typically welded onto the bottom of the shafts. Actuators are reversing electric motors controlled by a switch that push and pull to turn the sprocket, as would a push/pull cable like a Teleflex system.
Photos courtesy of Tim Escott
January, 2015